Purpose: Accurate volume estimation for time-resolved fan-beam CT (4DCT) and cone-beam CT (4DCBCT) is critical to planning and verification of many motion-sensitive radiotherapy applications, including stereotactic lung radiotherapy. We used a 4D torso phantom to acquire realistic respiratory images with 4DCT and 4DCBCT, and compared object size estimation of the two modalities.

Methods: A torso phantom consisting of a water-filled sphere surrounded by foam pellets was driven by a motion device capable of reproducing patient respiratory traces. Ten typical respiratory traces were imaged along with static and sinusoidal cases with clinically-relevant 4DCT and 4DCBCT techniques. 4DCT images were reconstructed in ten phases at 512x512x104 voxels and 0.977x0.977x2.50 mm³/voxel (GE Advantage). 4DCBCT images were reconstructed in ten phases at 128x205x205 voxels and 2.00x2.00x2.00 mm³/voxel (Elekta XVI). Objects were contoured in MIM 5.6.3 using threshold segmentation and volumes compared between image phases.

Results: Reference object size was 7.03 mL and 7.01 mL for the static CT and CBCT acquisitions, respectively. When averaged over all phases, 4DCT volume was 6.78+/-0.72 mL; however, variation between phases was large and significant in Kruskal-Wallis rank-sum tests (2.68--10.96 mL, p<0.001). 4DCBCT showed reduced accuracy of object volume estimation relative to the static case and 4DCT in paired sign-rank tests (μ=9.08+/-0.07 mL, p<0.001); however, 4DCBCT greatly improved consistency of estimation, as no significant differences were detected between phases (8.47--9.10 mL, p=0.4).

Conclusion: Variation in object volume between respiratory phases was dramatically inferior for 4DCT versus 4DCBCT, possibly due to fan-beam CT interplay effects or superiority of diaphragm sorting versus a linear surrogate. Consequently, radiotherapy planning or gating on individual phases of 4DCT datasets has potential to cause significant error. 4DCBCT did overestimate object size relative to 4DCT due to partial volume effects. As such, higher-resolution 4DCBCT presets should be investigated for patient verification imaging.